This invention relates to a method and apparatus for the testing of material in which the surface of the test piece is irradiated with monochromatic light.
The quality control of surfaces is of fundamental importance, particularly in the field of biomedicine for implant materials, in the field of pharmaceutical production for judging the homogeneity and composition of drugs, in the field of industrial chemistry for the manufacture of catalysts, and in general in the field of material engineering for the production of highly pure, corrosion-resistant materials.
Up to the present time in these fields, quality control, aside from true functional controls of individual specimens, has been effected only by sampling inspections with visual evaluation, with or without microscope, and at times at most by light-spectroscopic individual examination. Such techniques rely essentially on methods which operate with conventional emission or absorption spectroscopy. However, with a reasonable expenditure of personnel and material, such methods yield substantially only overall results which are not very specific; they either involve integration over large regions of the surface, or else their depths of penetration are too great, so that they provide only a mixed signal indicative of surface and interior conditions, or else they are not sufficiently informative with respect to the chemical constitution on a molecular plane, since it is only with difficulty that reflection-spectroscopic examinations in the infrared can be reproduced.
It is, furthermore, already known to study the chemical nature of, for instance, geological specimens by means of Raman spectroscopy. In this case, Raman spectra from different regions of the surface of the specimen are recorded and compared with the spectra of known chemical compounds. From the "Microprobe Mole" brochure published by the Yvon Company, there is known an apparatus with which it is possible not only to record Raman spectra of punctiform regions of microscopic specimens, but the specimen itself can be observed in the light of its characteristic Raman radiation. In this way, the local distribution of the substances to be analyzed can be determined.
The indicated apparatus is an extremely costly and expensive analytical instrument which is not suitable for routine investigation, due to its complicated construction. To observe Raman light on the wavelength characteristic for the specimen and to record the spectra, a special double monochromator is used, permitting continuous scanning over the desired wavelength range. The identification of substances to be analyzed is effected by having the operator compare their spectra with those of known compounds, and this requires a great amount of time.